Understanding distribution limits: contrasting hydrothermal physiology and drought vulnerability in two parapatric vipers in their hybrid zone

Climate change and extreme climatic events pose significant challenges to biodiversity. Studying species' physiological tolerances is required to predict their vulnerability and response to these threats, particularly at the margins of their distribution, where they are frequently at their environmental limits. While temperature constraints have attracted considerable interest, the combined effects of rising temperatures and aridification remain underrepresented in climate impact assessments, despite their synergistic role in intensifying physiological stress. Herein, we compared two parapatric vipers, Vipera aspis and Vipera latastei, which exhibit contrasting climatic niches and hybridise in their contact zone in northern Spain. Vipera aspis inhabits cooler, wetter environments, while V. latastei is adapted to warmer, drier habitats. First, we used open-flow respirometry to measure standard metabolic rate (SMR) and total evaporative water loss (TEWL) in pregnant females at three temperatures (15, 25 and 33°C). Vipera aspis exhibited higher SMR and TEWL than V. latastei and their hybrids, particularly at its preferred body temperature (33°C), reflecting its distinct temperate-adapted physiology. Second, we simulated a realistic drought (14 days) on neonates born from these females, manipulating both free-standing water and air water vapour deficit. In the drought-simulated treatment, mass loss and postnatal growth inhibition were most pronounced in V. aspis, while the hybrids exhibited on average an intermediate response between those of the two species. The warm- and dry-adapted V. latastei, therefore, exhibits greater drought tolerance under climate change scenarios, potentially providing a physiological advantage in the future dynamics of contact zones.